Among the various video display systems available in the art, an optical projection system is known to be capable of providing high quality displays in a large scale. In such an optical projection system, light from a lamp is uniformly illuminated onto an array of, e.g., M.times.N, actuatable mirrors, wherein each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodisplacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of, e.g., an optical baffle. By applying an electric signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of light reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
In FIGS. 1A to 1I, there are illustrated a method for manufacturing an array 100 of M.times.N thin film actuatable mirrors 101, wherein M and N are integers, disclosed in a copending commonly owned application, U.S. Pat. No. 5,757,539, entitled "THIN FILM ACTUATED MIRROR ARRAY FOR USE IN AN OPTICAL PROJECTION SYSTEM".
The process for the manufacture of the array 100 begins with the preparation of an active matrix 110 including a substrate 112, an array of M.times.N connecting terminals 114 and an array of M.times.N transistors (not shown). Each of the connecting terminals 114 is electrically connected to a corresponding transistor in the array of transistors.
In a subsequent step, there is formed a passivation layer 120, made of, e.g., phosphor silicate glass (PSG) or silicon nitride, and having a thickness of 0.1-2 .mu.m, on top of the active matrix 110 by using, e.g., a chemical vapor deposition (CVD) or a spin coating method.
Thereafter, an etchant stopping layer 130, made of silicon nitride, and having a thickness of 0.1-2 .mu.m, is deposited on top of the passivation layer 120 by using, e.g., a sputtering or a CVD method, as shown in FIG. 1A.
Then, a thin film sacrificial layer 140, made of, e.g., a PSG, and having a flat top surface, is formed on top of the etchant stopping layer 130 by using a CVD or spin coating method, followed by a chemical mechanical polishing (CMP) method, as shown in FIG. 1B.
Subsequently, an array of M.times.N pairs of cavities 145 is created in the thin film sacrificial layer 140 in such a way that one of the cavities 145 in each pair encompasses one of the connecting terminals 114 by using a dry or an wet etching method, as shown in FIG. 1C.
In a next step, a supporting layer 150, made of a nitride, e.g., silicon nitride, and having a thickness of 0.1-2 .mu.m, is deposited on top of the thin film sacrificial layer 140 including inside the cavities 145 by using a CVD method.
Thereafter, a second thin film layer (not shown), made of an electrically conducting material, e.g., platinum/tantalum (Pt/Ta), and having a thickness of 0.1-2 .mu.m, is formed on top of the supporting layer 150 by using a sputtering or a vacuum evaporation method. The second thin film layer is then patterned into an array of M.times.N second thin film electrodes 165 by using a dry etching method, wherein each of the second thin film electrodes 165 is electrically disconnected from other second thin film electrodes 165, as shown in FIG. 1D.
Then, a thin film electrodisplacive layer 170, made of a piezoelectric material, e.g., lead zirconium titanate (PZT), or an electrostrictive material, e.g., lead magnesium niobate (PMN), and having a thickness of 0.1-2 .mu.m, is deposited on top of the array of M.times.N second thin film electrodes 165 by using an evaporation, a Sol-Gel, a sputtering or a CVD method. The thin film electrodisplacive layer 170 is then heat treated to allow a phase transition to take place by using a rapid thermal annealing (RTA) method.
Subsequently, a first thin film layer 180, made of an electrically conducting and light reflecting material, e.g., aluminum (Al) or silver (Ag), and having a thickness of 0.1-2 .mu.m, is formed on top of the thin film electrodisplacive layer 170 by using a sputtering or a vacuum evaporation method, thereby forming a multiple layered structure 200, as shown in FIG. 1E.
In an ensuing step, as shown in FIG. IF, the multiple layered structure 200 is patterned into an array of M.times.N semifinished actuating structures 210 by using a photolithography or a laser trimming method, until the thin film sacrificial layer 140 is exposed, in such a way that each of the semifinished actuating structures 210 includes a first thin film electrode 185, a thin film electrodisplacive member 175, the second thin film electrode 165 and a supporting member 155.
In a subsequent step, an array of M.times.N conduits 190, made of, e.g., tungsten (W), is formed by using a lift-off method, wherein each of the conduits 190 is extended from top of the thin film electrodisplacive member 175 to top of a corresponding connecting terminal 114, thereby forming an array of M.times.N actuating structures 220, as shown in FIG. 1G.
Finally, the thin film sacrificial layer 140 is then removed by using an wet etching method using an etchant or a chemical, e.g., hydrogen fluoride (HF) vapor to thereby form an array 100 of M.times.N thin film actuatable mirrors 101, as shown in FIG. 1H.
There are certain deficiencies associated with the above described method for the manufacture of the array 100 of M.times.N thin film actuatable mirrors 101. The cavities 145 are, generally, formed into a rectangular shape by etching the thin film sacrificial layer 140 using a photoresist. As a consequence of the presence of the cavities, the thin film layers subsequently over layered on top of the sacrificial layer after the cavities are formed are not planar; they are provided with steps with sharp edges at the portions thereof coinciding with the cavities. The stress is concentrated at the sharp edges of the steps of the thin film electrodisplacive layer 170 coinciding with the cavities 145, the stress causing cracks to be formed when the thin film electrodisplacive layer 170 is heat treated to allow the phase transition to take place, which, in turn, deterimentally affects the structural integrity of the thin film actuatable mirror 101.